The generation of electric power is a complex matter, requiring the provision of widely varying amounts of power during various times. Fundamentally, however, maximum efficiencies are achieved by certain operating conditions that are steady state or near-steady state and thus are not compatible with these widely varying demands, particularly in the case of a steam turbine power plant. This problem can be addressed by providing various combinations of plants that are either on standby or running, and although running inefficiently, are smaller and thus waste less energy. Gas turbines are well suited for this latter use because of the ease with which they can be brought on-line. Such solutions, however, require sophisticated control systems to match the power demanded to the power output, bringing standby generators on line as needed, and otherwise managing the power grid.
Combined cycle power generation plants meet the needs of increased efficiency and flexibility because they blend the best features of peaking and base-load generation by combining a steam turbine system with one or more gas turbines. As mentioned above, gas turbines have short start up times and respond well to changes in power demands; they are, however, relatively inefficient for power generation in simple cycle applications. Steam turbines are not well-suited for fast start up and for response to varying demand. Combined cycle plants can achieve better efficiencies by utilizing the waste heat from the exhaust of gas turbines to generate steam for the steam turbine.
Although the heat from the gas turbines is currently used to a certain extent, it is always desirable to further improve the efficiency of the combined cycle plant. Thus, it is an object of the present invention to provide an improved design for combined cycle plants that makes fuller use of the heat generated by one or more gas turbines.
The overall efficiency of gas turbines is a function of the compressor and turbine efficiencies, ambient air temperature, turbine inlet temperature, overall pressure ratio and the type of cycle used. Certain of these conditions are not controlled by the plant layout but by the equipment design, while others are not readily controlled. It is possible, however, to control the temperature of the gas entering the combustor. The higher this temperature, the higher the efficiency of the turbine cycle. Thus, it is a further object of the present invention to increase the temperature of the gas entering the combustor.
Finally, environmental concerns are more important today than ever. The use of a combined cycle plant generates pollutants primarily in the form of NO.sub.x and carbon monoxide (CO). Thus, it is another object of the present invention to provide a system whereby the efficiency of the plant is increased, yet the production of pollutants is reduced.
U.S. Pat. No. 5,133,180--Horner et al. discloses that reformers or chemical recuperators can be used to produce a fuel having significant quantities of hydrogen and water vapor, which is injected into a gas turbine engine to increase performance and produce low emissions. A semi-open cycle gas steam turbine that uses a recuperator is also disclosed in U.S. Pat. No. 5,271,215--Gulliet. A method of increasing the efficiency of a gas turbine and reducing emissions by heating compressed air by passing it through a flue gas heat exchanger is disclosed in U.S. Pat. No. 5,319,925--Hendriks, et al.